1. Field of the Invention
The invention relates generally to microwave antenna systems, and in particular to a broad bandwidth waveguide lens for providing a constant phase aperture plane.
2. Description of the Prior Art
Microwave scanning antenna systems are generally known in the prior art. So, too, are waveguide lenses for use in conjunction with such antenna systems. A constrained microwave lens or waveguide lens is comprised of an array of waveguide sections, and is utilized to produce a plane phase front at the aperture. The prior art lenses having the broadest bandwidth are the ones having an equal time delay of all the rays from the focal point to the aperture, regardless of frequency. One such broad bandwidth lens is the "bootlace" lens. Lenses that deviate most from the equal time delay principle have narrowest bandwidth, and the bandwidth increases as equal time is approached. Conventional waveguide lenses based upon the principle of equal phase delay are very narrow in bandwidth because there is a very large difference in time delay between the central and edge rays. Since the index of refraction of waveguides is less than unity, the lens' outer surface is concave in contour, with the largest lengths of waveguide at the edge of the lens where the path is inherently longest.
Conventional waveguide lenses may be "zoned" to either increase bandwidth or minimize weight. In "zoning" the waveguide lens is divided into concentric annular rings or zones. Incrementally varying portions of the individual waveguides are removed within each annular zone. Zoning the lens removes the waveguide in increments of differential phase between the waveguide and free space at the zone steps. This process diminishes the difference in time delay among the various rays; hence bandwidth is improved. Zoning for minimum weight as is conventionally done produces an aperture phase distributed at off-design frequency which is sawtooth with a mean value that increases quadratically from the center of the lens to the edge. Coulbourn has been able to improve the bandwidth of a zoned lens by adding thickness to the central portion of the lens. The added thickness allows the number of zones to be increased and makes the time delay nearly equal at discrete points in each zone. The aperture phase distribution of the Coulbourn lens at frequency off the design frequently is sawtooth, with a mean error of zero. Both the zoned and the Coulbourn lenses are difficult and expensive to manufacture due to the zoning. Also, such lenses do not lend themselves easily to the use of a radome, due to their uneven and complex surfaces.
Another type of lens is the constant thickness waveguide lens wherein the waveguides have a constant thickness, and phase correction of off axis-rays is achieved by means of phase shifters inserted into the waveguide elements. Because the phase shift is constant with frequency, the constant thickness half wave plate lens is narrow band.